Method and apparatus enabling thermal energy recovery in combustor operation

ABSTRACT

The specification describes a method and arrangements for operating fluidized bed combustors in particular boilers. In the described method and arrangements thermal energy carried away from the combustor by the flue-gas emitted therefrom is recuperated by passing the flue-gas through a scrubber in which it is scrubbed with water. The scrubbing water subsequently being used to pre-heat fluidizing gas fed to the fluidized bed in the combustor and/or working fluid fed to the boiler. Additional description is given of using a heat exchanger through which both flue-gases from the combustor and the working fluid fed to the combustor are fed such that the heat carried away from the combustor in the flue-gases is given directly to the working fluid fed to the combustor. In the described method and arrangements the fluidizing gas may be charged with water vapor and/or steam easing control of the bed.

DESCRIPTION TECHNICAL FIELD

The present invention concerns methods of operating combustors andcombustor apparatus; the invention is particularly, but not exclusively,concerned with recovering at least part of the thermal energy normallylost when operating a fluidised bed combustor.

BACKGROUND ART

Fossile fuel combustors discharge flue-gases at elevated temperatures(the flue-gases from steam raising boiler plant for example, arenormally discharged at a temperature somewhat above the temperature ofthe steam produced in the boiler) usually via a tall chimney to theatmosphere.

This method of operating produces an air draft which disperses noxiousgases and alleviates acid condensate and its corrosive effects. Themethod does not however prevent atmospheric pollution; and the dischargeof hot gases to the atmosphere means that there is a considerablethermal loss from the combustor with the result that the overalloperating efficiency of the combustor is lower than would otherwise bethe case.

DISCLOSURE OF INVENTION

One aspect of the present invention provides a method of operating afluidised bed combustor including a bed of inert particulate material towhich gas is fed to fluidise the bed and support combustion of fuels fedthereto, the method comprising scrubbing with water flue-gas passingfrom the combustor and subsequently transferring heat from the scrubbingwater to the fluidising gas fed to the combustor.

A second aspect of the present invention provides a method of operatinga boiler comprising the steps of scrubbing the flue-gas issuing from theboiler with water and using that water to pre-heat the working fluid fedto the boiler.

A third aspect of the present invention provides a fluidised bedcombustor including means for passing fluidising gas to a bed of inertparticulate material in the combustor, the combustor comprising scrubbermeans for scrubbing with water flue-gas passing from the combustor andheat transfer means for transferring heat from the water used to scrubthe flue-gas to the fluidising gas fed to the bed in the combustor.

A fourth aspect of the present invention provides a boiler includingscrubber means for scrubbing with water flue-gas passing from the boilerand thermal transfer means for transferring heat from the water used toscrub the flue-gas to the working fluid fed to the boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described withreference to the accompanying drawings which show at:

FIG. 1 a schematic outline of a fluidised bed boiler arrangementembodying the present invention, and at

FIGS. 2, 3, 4 and 5 illustrate various modifications which may be madeto the arrangement of FIG. 1

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIG. 1 which shows a fluidised bed shell type boiler10 to an inlet 11 of which a working fluid, water, is fed from a source12. Heater water, or steam, is taken from the boiler via an outlet 13.The boiler 10 shown in FIG. 1 is a shell boiler but it will beappreciated that any other kind of boiler may be used.

The gaseous products of combustion of the boiler pass therefrom via aflue 14. Gas to fluidise the bed in the boiler is fed to the boiler viaan inlet 15. The fluidising gas comprising a mixture of ambient airsucked in by a fan 16 and recycled flue-gas taken from the end of astack 17 connected to the outlet flue 14 of the boiler as shown.Flue-gas is extracted from the stack 17 by a recycle gas fan 18 asshown.

Mixture of recycled flue-gas and of fresh air fed to the boiler iseffected in a plenum chamber 20; the ratio of air and/or flue-gas passedto plenum chamber 20 is controlled by operation of respective valves 21and 22 as shown. The fluidising gas from the chamber 20 is fed to thematerial of the bed 23, in the boiler 10, via an array of sparge pipes24 located within the bed material as shown.

Air from fan 16 is fed to plenum chamber 20 via an enclosed annularvolume 25 surrounding a scrubber 26 in the base part of stack 17.Flue-gas from the outlet 14 of boiler 10 enters scrubber 26 via an inlet27 and passes up, through scruber 26, to the upper part of stack 17.Whilst passing up scrubber 26 flue-gas from flue 14 meets a downwardlydirected flow of water issuing from a sparge-element or elements 28located immediately below the outlet of stack 17.

Water from the element or elements 28 passes down the scrubber 26 tocollect in a sump 29. Water is withdrawn from sump 29, via a pipe 30 andpump 31, and passed to a generally circular sparge element 32 located inan upper part of annular volume 25. Water falls from element 32 downannular volume 25 and is extracted at the bottom of that volume, via apipe 33 and pump 34, and passed to the sparge element or elements 28 asshown.

It will be appreciated that the water falling down scrubber 26 andannular volume 25 is passing effectively around a closed loop. As waterfalls down scrubber 26 it contacts the heated flue-gas moving upwardlyand warms. The water so heated collects in sump 29 and is then passedvia element 32 to the volume 25 in which it is intermixed with airpassing therethrough. The water, heated by the flue-gas as it falls downscrubber 26, therefore gives up at least part of the heat it hascollected to the air that is passing, through volume 25 on its way tothe bed 23 in boiler 10. At the same time part of the water falling downthe annular volume 25 may be entrained with the air passing therethroughand carried to plenum chamber 20. Additional "make-up" water may beprovided to the closed loop at an inlet 35 as shown.

It will be seen that the apparatus described with reference to FIG. 1provides that the flue-gas passing from the boiler is scrubbed before itleaves the stack 17 thereby reducing thermal and chemical pollution ofthe atmosphere, also it will be seen that the air from the inlet fan 16is warmed by contact with the heated scrubbing water falling downannular volume 25 (and may in fact be charged with water vapour) priorto its being mixed with recycled flue-gas in the plenum chamber 20 andpassed to the fluidised bed 23 in boiler 10.

As shown in FIG. 1 the sump 29 of stack 17 includes outlet means 36enabling the sludge or slurry of dirt and other particles entrained bywater falling down scrubber 26 to be removed from the sump. Additionalmeans (not shown) may be provided to enable water to be added to thesludge or slurry in sump 29 both to reduce its acidity and to ease itsremoval from the sump.

Water collected from the sump 29 and passed to the sparge element 32 isused to pre-heat the working fluid passing from source 12 to the boilerinlet 11. As shown the pipe 30 connecting pump 31 with element 32 isprovided with a branch 37 leading to a shell-and-tube heat exchanger 38.After passage through the heat exchanger 38 the water bled-off in branch37 is remixed with the main flow of water from the pump 31. In the heatexchanger 38 water from sump 29 gives up at least part of the heat itcarries to the water being passed to the inlet 11 of the boiler.

Apparatus embodying the present invention may also provide that theflue-gas passing from the boiler is (in addition to the cooling whichtakes place in the scrubber 26) separately cooled.

In the alternative arrangement of FIG. 2 flue-gas passing from theboiler 10 is passed through a heat exchanger 39 prior to being passed tothe inlet 27 of the scrubber 26. In heat exchanger 39 heat from theflue-gas is given up to water passing from the source 12 to the inlet 11of the boiler 10.

In a second alternative arrangement (see FIG. 3) a heat exchanger 40 islocated within the scrubber 26 increasing the cooling of the flue-gaspassing through scrubber 26 and enabling water passing from the source12 to be pre-heated prior to its being fed to the boiler 10.

A further modification to the design shown in FIG. 1 is illustrated inFIG. 4 in which it will be seen that the recycled flue-gas is mixed withthe air from the inlet fan 16 prior to passage of the fluidising gasthrough annular volume 25. Such an arrangement may be advantageous whenit is particularly desired that the fluidising gas fed to the bed 23 inboiler 10 carry a high proportion of water vapour.

In the modified arrangement of FIG. 5 the recycling of flue-gas at atemperature in excess of 100° C. is enabled. This arrangement providesthat the flue-gas is drawn off from the lower part of the stack via aconduit 50 and fan 18 (or even as shown in dotted outline from the flue14 directly) prior to its being cooled by passage upwardly through thescrubber 26. Such an arrangement enables the mixture of gases fed to theboiler 10 to fluidise the bed 24 to carry steam (not being limited aswhen the fluidising gas temperature is below 100° C. to carrying vapourat 100% relative humidity). The proportion of steam carried by themixture of fluidising gas in the arrangement of FIG. 5 may be augmented,for example by water injection as shown at 51 under the control of avalve 52 bleeding pre-heated water from the supply to inlet 11. Theadvantages of the arrangement of FIG. 5 are that the heat-transfer andbed cooling are improved by making use of the higher specific heat ofsteam, the water-gas reaction and some dissociation within the bed 23.

As described the flue-gas may be drawn from the flue 14, the scrubber 26or a section of the scrubber where the temperature of it is in excess of100° C. The flue-gas not so withdrawn may be exhausted to atmosphereafter passage upwardly through the scrubber 26 (FIG. 1) either alone orin combination with a heat exchanger such as at 39 in FIG. 2 or 40 inFIG. 3.

It will be seen from all the described arrangements that the thermaltransfer from the flue-gas to the scrubbing water, and the subsequentthermal transfer from that water to the fluidising/combustion air and/orrecycled flue-gas fed to the fluidised bed in the combustor 10 givesmany advantages over the known arrangements.

Particular advantages which arise are that the flue-gas is scrubbedprior to its being ejected from the chimney with a subsequent reductionin atmospheric pollution, and that the flue-gas is cooled as it passesthrough the scrubber 26 (and possibly separate heat exchangers 29 and/or40) so that thermal pollution is reduced and at least part of the heatnormally wasted by discharge of the flue-gas to the atmosphere is savedand returned to the boiler 10. Other advantages are that the gas fed tothe fluidised bed 23 in the boiler 10 to aid combustion therein ischarged with water vapour or steam enabling control of the boiler to bemore readily effected than would otherwise be the case. It will beappreciated that the water or steam added to the gas passing to thefluidised bed of the boiler is at a higher temperature than theatmospheric gas passed to the boiler thereby reducing the thermal dropupon introduction of the gas to the boiler.

It is necessary; in the apparatus and a method of the present inventionfor there to be intimate thermal contact between the flue-gas and thewater which is used to scrub it and this is best obtained, as describedin the illustrated embodiments, by use of spray chambers or the like inwhich the water is passed to the scrubber chamber 26 by an array ofsparge elements having relatively small apertures through which watercan escape. Operation in this manner results in a scrubbing action asthe water passes down the scrubbing tower 26 which percipitates dust andfly-ash carried away from the boiler 10 by the flue-gases. In additionthis intimate thermal contact dissolves polluting constituents, forexample sulphur dioxide, which are the combustion products of thesulphur particles within hydrocarbon fuels used in the boiler.

The descending water falling through scrubber 26 to the bottom of thestack 17 means that solids entrained with the water can settle as asludge or slurry in the sump 29 and, as noted above, water may be addedto this sump to ease removal of the sludge and reduce its acidity.

The water taken from the sump 29 is pumped to the sparge element 32enabling the heat the water has gained in passing through the scrubberto be given up to the gas fed to the bed 23 and moisture to be carriedto the bed 23. This increases the thermal transfer rate beyond thatwhich could be achieved simply by heating the gas. In the arrangement ofthe present invention this is achieved by using relatively high airvelocities for the air which passes through the volume 25.

When, as described in the present arrangements, the boiler includes afluidised bed the high moisture content of the gas fed to the bed mayimprove the operating efficiency of the boiler in two particularrespects, firstly the water carried with the combustion air improvesin-bed cooling, and secondly the water may provide that a higher thermaloutput be obtained (by allowing the water carried by the gas to beturned into super-heated steam at the temperatures in the bed and alsodue to the water-gas reaction and marginally by disassociation). Thesuper-heated steam which is formed in the bed from the water carried inwith the fludising gas passes through the boiler and, via the flue,re-enters scrubber 26 wherein it condenses. Condensation of steam in thescrubber should be made as complete as possible by reducing the gasvelocities and increasing the contact times between the flue-gases andthe water so that steam does not escape to the atmosphere although wehave found that discharge of moisture at something approaching thesaturation point is unavoidable.

Cooling of the fluidised bed 23 medium, which is usually an inertparticular material such as sand, may be achieved by a number ofdifferent methods such as for example by recycling the flue-gas. Whenusing this method and apparatus the effectiveness of the cooling isincreased by cooling the flue-gas in the scrubber 26 (and in the heatexchanger 39 and/or 40).

An advantage found when using the apparatus and method of the presentinvention is that the reduced temperature of the recycled flue-gas whichis abstracted to be re-passed to the boiler (due to its being cooled inscrubber 26) enables a reduction to be made in the fan size over thatwhich would otherwise have to be used. That is to say the width anddiameter of the recycle flue-gas fan 18 may be reduced and/or the speedof the fan may be reduced.

It is suggested that operation of the combustion apparatus including thedescribed features improves the efficient use of valuable fuel and cutsthermal losses to something less than half of the values at presentobtaining in comparable steam raising plant. The recuperative heatrecovery by means of recycling water, which although described as beingused with sparge pipes may be used with any other convenient element,also achieves precipitation of noxious compounds (thereby reducingatmospheric pollution); grit and dust (that may be carried away in theflue-gas) and removes the need for cyclone filters (necessary in certainarrangements presently available).

INDUSTRIAL APPLICABILITY

Although the above description is made with reference to a fluidised bedboiler it will be appreciated that the described principles may beapplied to any form of combustor whether or not it includes a fluidisedbed; it will further be seen that various modifications may be made tothe described arrangements without departing from the scope of thepresent invention.

We claim:
 1. A method of operating heating apparatus including afluidised bed of inert particulate material to which gas is fed tofluidise the bed and support combustion of fuel fed thereto, the methodcomprising using a mixture of air and recycled flue gas the proportionsof which may be varied to fluidise the bed of inert particulatematerial, scrubbing flue gas passing from the heating apparatus withwater and bringing at least a part of the air fluidising the bed intocontact with the scrubbing water to enable said fluidising gas toentrain and carry water vapour and/or steam to said fluidised bed to aidcontrol of the temperature of the bed, said fluidising air being fed tothe fluidised bed by passing at least a part thereof through a chamberto which the water used to scrub the flue-gas is also passed.
 2. Amethod according to claim 1, in which additional water is injected intothe flow of fluidising gas fed to the heating apparatus.
 3. A methodaccording to claim 1 wherein the water used to scrub the flue gas ispassed through a heat exchanger in which it heats the working fluid fedto the boiler.
 4. The method according to claim 1 wherein said heatingapparatus is a combustor.
 5. The method according to claim 1 whereinsaid heating apparatus is a boiler.